The y-intercept in 2-D space is the point where graph of a function or relationship intercepts the y-axis of the coordinate system.

If the function is specified in form y = f(x), the y-intercept is easy to find by calculating f(0). For example, in linear equations that are in the "slope-intercept" form of y = mx + b, the value of b is the y-intercept. In general, in polynomial expressions of form y = P(x),where P is a polynomial, the last (or numerical term) is the y-intercept of the polynomial.

If the relationship is in the form f(x,y) = 0, or in the form of parametric equations, the corresponding equation (equations) must be solved. As a result, some 2-D mathematical relationships such as circles, ellipses, and hyperbolas can have more than one y-intercept.

The notion may be extended for 3-D space and higher dimensions, as well as for other coordinate axes, possibly with other names. For example, one may speak of the I-intercept of the I/V-characteristic of, say, a diode.